78 Frequency Converters and Motors 3 Frequency Converters and Motors In the previous chapters, the motor and the FC were each presented in isolation. This chapter explains the interaction between the two components. 3.1 Basic Principles 3.1.1 U/f Operation and Field Weakening The major technical characteristics of a motor are found on its nameplate. The information shown is very relevant for the electrical installer, because values for voltage, frequency and full load current are given, but important information for the mechanical design is missing and can be found in the datasheet, catalogue, or by direct contact to the motor manufacturer.
This mechanical design information includes data related to motor start and intermittent operation, and also the available torque at the motor shaft. The shaft torque is easy to calculate from the nameplate data. For a given load, the following expression applies: T = = = P u 9550 n k u V u I f n u — 3 u V u I u cos M u 9550 f u 60/p u (1-s) This results in the principle relation: T ~ u I V f This relation is utilised in voltage source FC’s which maintain a constant ratio between the voltage (U) and the frequency (f ) . This constant ratio (U/f ) determines the magnetic fl ux density (Φ) of the motor and is determined by the motor nameplate data (for example, 400 V/50 Hz = 8 [V/Hz]).
The constant fl ux density ensures optimum torque from the motor. Ideally the ratio 8 [V/ Hz] means that each 1 Hz change in the output frequency will result in an 8 V change in the output voltage. This way of controlling the output values of the FC is called “U“ to “f ” characteristic control.